The present invention relates to a biaxially oriented laminated film which is composed of a surface layer of a polyester, an intermediate layer of a polyamide and a heat-sealing layer of a polyolefin and which has excellent stretching processability, heat-sealing property and packaging property, and shows a good transparency after heat sterilization.
Polyesters are generally excellent in heat resistance and packaging property, but they have a low impact strength. To improve the impact strength, polyesters are subjected to a stretching processing. Since ordinary polyesters have a high crystallinity, it is necessary to orient them with great force, which requires a large-sized stretching machine. A polyamide is laminated on a polyester in order to further improve the mechanical strength. Since the crystallizing rate of polyamides is also nigh, it is difficult to subject the polyamides to an ordinary stretching processing. In this way, a laminated film consisting of only these two resins is not easy to stretch, and the productivity of such a stretched film is poor at present. A film produced from these resins must be stretched with great force and naturally requires after-treatments such as heat setting and heat treatment which may be the cause of the lowering of the interlaminar strength.
Heat sealing is generally adopted as a sealing method in automatic packaging. Even if the above-described laminated film is managed to be stretched, since the film is oriented, it is difficult to heat-seal. As a countermeasure, polyolefins are used as a heat-sealing layer. In this case, since the respective resin layers have different degrees of orientation from each other, an adhesive layer is inserted therebetween, but it is still impossible to obtain a high interlaminar strength and, hence, a sufficient sealing strength. Especially, in the case of operating a packaging machine at a high speed, the sealing strength of the film is apt to be influenced by the difference in the degree of orientation. If the polyolefins are used as a surface layer, the polyolefins are apt to adhere to the sealing bar and makes automatic packaging at a high speed difficult.
If the mutual balance of the degree of orientation of the polyester, polyamide and polyolefin is lost due to stretching, the laminated film shrinks during the bagmaking, so that it is difficult to bundle the bags and the transparency of the films after heat sterilization is greatly lowered.
With respect to polyester films, Japanese Patent Application Laid-Open (KOKAI) No. 58-175658 (1983) discloses a biaxially oriented film produced by laminating a polyamide copolymer film as a heat-sealing layer, having a melting point of not higher than 150.degree. C. at least one side of a polyethylene terephthalate film and substantially heat-treating the laminated film.
Japanese Patent Application Laid-Open (KOKAI) No. 60-232948 (1985) discloses a polyester laminated film which is composed of a polyethylene terephthalate layer and a copolyester layer modified by isophthalic acid, and which is heat-treated after the film formation and the stretching processes.
Japanese Patent Publication No. 55-3137 (1980) discloses a composite film produced by laminating a polyethylene terephthalate film (A) and a polymer film (B) which has a stretching temperature in the range of 70.degree. to 120.degree. C. and which is other than (A), through an adhesive layer containing one selected from the group consisting of an urethane-modified polyester, polyalkylene imine, a alkylated titanium and a self-crosslinking acrylic resin as the main ingredient, and uniaxially or biaxially orientating the laminate.
These biaxially oriented laminated films, however, cannot be said to have sufficiently solved the problems such as stretching processability, heat-sealing property and transparency.
As a result of studies undertaken by the present inventors to solve the above-described problems of a stretched polyester film, it has been found that by laminating on a specific polyester layer as a surface layer, a specific polyamide layer as an intermediate layer and a polyolefin layer as a heat-seal layer so that the thickness of the polyester resin layer is lower than that of the polyamide layer, the thus obtained biaxially oriented film has excellent stretching processability, heat-sealing property and packaging property, and shows a good transparency after heat sterilization. On the basis of this finding, the present invention has been achieved.